Method and apparatus for making resistivity measurements in a wellbore

ABSTRACT

During drilling of an earth borehole, resistance measurements may be made at the drill bit through use of a bottom hole assembly that includes a drill bit having a sensor, such as an electrode, located generally at an exterior surface of the drill bit. The current will be induced in the formation from multiple transmitters, at least one of which will be supported on, or very close to the drill bit. Connection mechanisms are described that enable the releasable engagement of electrical conductors to circuitry within the drill bits. The obtained resistivity measurements at the drill bit can be used for many purposes, including formation imaging and geosteering of the drilling operation.

BACKGROUND OF THE INVENTION

The present invention relates generally to methods and apparatus formaking resistivity measurements in a wellbore, and more particularlyrelates to such methods and apparatus for making resistivitymeasurements proximate a drill bit, such as in a logging while drillingor measurement while drilling environment. Such resistivity measurementsproximate the drill bit may be used for various types of analysis,including in some instances imaging of the formation and geosteering ofthe drilling operation.

As is well known, resistivity measurements may be used both to evaluateformation properties, and to identify bed boundaries to assist insteering of a drilling operation. In trying to evaluate formationproperties, one problem that is faced is measurement errors resultingfrom fluid invasion of the drilling mud and associated filtrate into theformation. As this invasion surrounding the wellbore is a function ofseveral factors, including time, it would be desirable to measureresistivity at the drill bit to minimize the time component of theformation invasion.

Because of factors such as the above, the desirability of makingresistivity measurements at, or ahead of, the drill bit during adrilling operation has been proposed. However, the actual constructionand configuration of a bottom hole assembly, including a drill bit, tofacilitate those resistivity measurements presents substantialchallenges. These challenges include how to provide electricalcommunication to and from components in the drill bit, particularly in aconfiguration that is suitable for the realities of drilling operations,and particularly in systems wherein geosteering mechanisms are employed.Other challenges include how to configure the sensors to obtain thehighest quality measurements at the drill bit; how to include thenecessary functionality in the drill bit in a manner which iscost-effective; as well as additional challenges. Thus, notwithstandingrecognized benefits to making such measurements at the drill bit,limited practical configurations have been proposed to make thosemeasurements.

Accordingly, it would be desirable to have a practical bottom holeassembly, and an included drill bit, that enables the taking ofresistivity measurements at or near the drill bit, or ahead of the drillbit, and which addresses at least some portion of one or more of theabove challenges.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in more detail, therein are depictedvarious embodiments demonstrating examples of apparatus in accordancewith the present invention.

FIG. 1 depicts an example bottom hole assembly as used in a drillingoperation, depicted within a wellbore.

FIGS. 2A-B depict a drill bit assembly of a type similar to thatdepicted in FIG. 1, depicted substantially in vertical section, with theview of FIG. 2B rotated around the longitudinal axis of the depictedbit, relative to the view of FIG. 2A.

FIGS. 3A-B depict exemplary button electrodes of one type suitable foruse with the present invention, depicted in concentric circular form inFIG. 3A, and in concentric oval form in FIG. 3B.

FIG. 4 depicts an upper portion of a geosteering assembly as may be usedin the drilling assembly of FIG. 1.

FIG. 5 depicts an alternative configuration for a drill bit as may beused in a drilling assembly such as that of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following description refers to the accompanying drawings thatdepict various details of examples selected to show how the presentinvention may be practiced. The discussion addresses various examples ofthe inventive subject matter at least partially in reference to thesedrawings, and describes the depicted embodiments in sufficient detail toenable those skilled in the art to practice the invention. Many otherembodiments may be utilized for practicing the inventive subject matterthan the few illustrative examples discussed herein, and many structuraland operational changes in addition to the alternatives specificallydiscussed herein may be made without departing from the scope of theinventive subject matter.

In this description, references to “one embodiment” or “an embodiment,”or to “one example” or “an example” mean that the feature being referredto is, or may be, included in at least one embodiment or example of theinvention. Separate references to “an embodiment” or “one embodiment” orto “one example” or “an example” in this description are not intended tonecessarily refer to the same embodiment or example; however, neitherare such embodiments mutually exclusive, unless so stated or as will bereadily apparent to those of ordinary skill in the art having thebenefit of this disclosure. Thus, the present invention can include avariety of combinations and/or integrations of the embodiments andexamples described herein, as well as further embodiments and examplesas defined within the scope of all claims based on this disclosure, aswell as all legal equivalents of such claims.

As will be described herein, resistivity measurements may be made at thedrill bit through use of a bottom hole assembly (BHA), that includes adrill bit having a sensor, such as an electrode, located generally at anexterior surface of the drill bit. In embodiments where the resistivitymeasurements will be used for geosteering, the drilling assembly willpreferably include a geosteering assembly, such as a bent sub or anon-linear drive mechanism, coupled in an operative arrangement with amotor for rotating the drill bit, as is known in the art. Additionally,in order to achieve a desired depth of sensitivity of the resistivitymeasurements, it will be preferable to have at least one transmitter,such as a toroid antenna, located a predetermined distance from the oneor more sensors at the drill bit, such as an electrode or multipleelectrodes. In the presently described embodiments, for example, byplacing a transmitter toroid antenna supported proximate the upper endof a geosteering assembly, a spacing of approximately 22 feet or morefrom the drill bit electrode may be obtained.

As will be described herein, there are multiple desirablefunctionalities that may be incorporated into the bottom hole assemblyof the present system. One such desirable functionality is the abilityto communicate electrically with structures uphole in the drillstring.The example drill bit design described herein includes an electricalconnector that will connect through a conductive cable with acomplementary connector above the drill bit, such as proximate the topof the geosteering assembly. This connection can provide communicationthrough the one or more electrical conductor to an upper electricalstructure, for example an upper transmitter toroid antenna locatedproximate the top of the geosteering assembly, as discussed above.Another desirable functionality is an electronics assembly housed withinthe drill bit, which will preferably be in electrical communication withthe releasably engageable electrical connector. The electronics assemblymay either be constructed to be disposable with the drill bit, or may beconfigured for easy removal from the remainder of the drill bitstructure after the drill has been used. Finally, another desirablefunctionality is providing a toroidal antenna, or another antennaconfiguration, supported proximate the drill bit, and preferablysupported directly on the drill bit, within relatively close proximityto the one or more electrodes on the drill bit. For example, it will bepreferred to have this toroidal antenna within no more than 36 inchesfrom the bottom-most surface of the drill bit, and preferably within 18inches of that surface. These and other beneficial functionalities willbe discussed further in specific reference to the Figures herein.

Referring now to FIG. 1, the figure depicts a portion of a drillingassembly 10 disposed within a well or borehole 12 extending through anearth formation 14 in a conventional operational environment. Drillingassembly 10 terminates in a bottom hole assembly 16, which includes anearth boring assembly which includes a drill bit 26. In addition to thedrill bit, the earth boring assembly may optionally include one or moreother devices coupled directly or indirectly to the drill bit, forexample a sleeve, a connecting sub (as depicted at 42), or a“piggy-back” stabilizer In some relatively simple embodiments,particularly ones in which geosteering is not used, a bottom holeassembly will include a drill collar, and potentially other structures,coupled above a drill bit assembly, and the drill pipe will engage thebottom hole assembly. However, with many geosteering configurations, thelength of the drilling assembly beyond the point of deflection of thedrilling axis is a factor to be minimized. Thus, in drilling assembliesintended for use in geosteering applications, the drill bit will oftenrepresent the entire bottom hole assembly (and the entire earth boringdevice), below the geosteering assembly.

For the present illustration, drilling assembly 10 will be described asone suitable for use in geosteering operations, and thus includes ageosteering assembly, indicated generally at 22, with an output drive40, which is coupled through an optional connecting sub 42 to drill bit26. Drill bit 26 can also, as noted above, be coupled directly to outputdrive 40 of geosteering assembly 22. Geosteering assembly can be anydesired configuration as known in the art, including bent subs, etc. Inmost such configurations, the geosteering assembly will be used inconjunction with a downhole motor, and will be coupled (directly orindirectly) to the output or drive shaft of a downhole motor, such as amud motor 18, above the geosteering assembly. One desirable geosteeringassembly will be one such as that marketed by Sperry Drilling Services,a Halliburton company, as the Geo-Pilot® rotary steerable system,wherein the drill bit is driven by an output shaft from the mud motorcoupled to (and through) the geosteering assembly, and wherein the shaftis deflected from the longitudinal axis of the tool string between twoeccentric bearings, to drive the bit in a desired direction relative tothe nominal longitudinal axis of the drilling assembly.

Drilling assembly 10 also supports a first toroid antenna 20. Toroidantenna 20 may be contained in a separate sub, or as will be describedherein, may be incorporated into an existing component in drillingassembly 10. In the example embodiment described herein, toroid antennais incorporated into a tubular member in an upper portion of thedescribed geosteering assembly. As will be described in more detaillater herein, the geosteering assembly will also include an internalstructure to enable electrical communication between toroid antenna 20and other components further down in drilling assembly 10.

In this example, the geosteering assembly 22 will couple directly todrill bit assembly 26. For ease of reference, the drill bit assembly 26will just be referred to herein as “drill bit 26.” However, as will beapparent from the discussion below, and particularly the description ofFIGS. 2A-B, drill bit 26 is an assembly of components. Drill bit 26includes a bottom cutting surface (or “bit face”) including a pluralityof cutting elements indicated generally at 30. Cutting elements 30 aredepicted in the example of drill bit 26 as stationary cutting elementssuch as those formed of polycrystalline diamond (PDC cutting elements).Alternatively, drill bit 26 could have any configuration of cuttingelements known in the art, including for example, a plurality of movingcutters, as found on roller cone drill bits.

Additionally, drill bit 26 is depicted as an extended gage bit, formedby a stabilizing section 32 formed of a plurality of spirally-formedblades 34, having a maximum lateral dimension equal to the gagedimension of the drill bit. Drill bit 26 also includes at least oneelectrode placed in a location to provide reasonably reliable contact(both physical and electrical), with the formation. Thus, such anelectrode may be placed at various locations, including proximate thebit face 28 of drill bit 26, such as between rows (or otherdistributions) of PDC cutting elements 30, or at other locations. Inthis example, drill bit 26 includes a plurality of electrodes 36, andthe electrodes are distributed in longitudinally-offset locations onstabilizer blades 34. In that position, because the stabilizer blades 34extend to the gage dimension of the bit, the electrodes will beimmediately adjacent sidewalls of borehole 12, and typically in contactwith those sidewalls during rotation of the drill bit.

As will be described in more detail later herein, in this exampleembodiment, drill bit 26 also includes a second toroid antenna supportedbeneath protective coverings, and preferably immediately above thestabilizing section of drill bit 26. Also as will be described in moredetail later herein, drill bit 26 includes an internal detachableelectrical connection assembly facilitating electrical communication todrill bit 26, including in many embodiments, to second toroid antenna 38as well as to additional circuitry located within drill bit 26. Whileplacement of toroid antenna 38 on the drill bit is a highly desirableconfiguration, it will be possible, based on the disclosure herein, toplace the second toroid antenna in another component, such as a sub (forexample), coupled (directly or indirectly) to the drill bit (again,preferably within 36 inches of the bottom-most surface of the drill bit,and most preferably within 18 inches). In that case, the electricalconnection assembly could be supported within the component supportingthe second toroid. This electrical connection may be through a singlemono-conductor, or may be through multiple conductors. The electricalconnection will be described herein as being through a single conductor,flexible cable 44, schematically depicted in phantom in FIG. 1. In FIG.1, for clarity, flexible cable 44 is depicted as extending through thecenter of the drilling assembly. As will be apparent to those skilled inthe art, where the drilling assembly includes a geosteering assemblywith a non-linear longitudinal axis, the cable will not always becentralized along the full length of that geosteering assembly, but willextend linearly therethrough.

Referring now to FIGS. 2A-B, the figures depict drill bit 26 in greaterdetail, and partially in vertical section; with the view of FIG. 2Brotated around the longitudinal axis of the depicted drill bit 26,relative to the view of FIG. 2A. Elements depicted in FIGS. 2A-B thatwere previously identified in the discussion of FIG. 1 have beennumbered similarly in FIGS. 2A-B. Drill bit 26 includes a body member(or “bit upper section”) 202 which may typically be formed of steel.Drill bit 26 also includes a cutting assembly, illustrated generally at204, that supports a distribution of cutting elements 30 which willengage and cut the formation. While cutting assembly may be of unitaryconstruction with body member 202, in this embodiment, cutting assembly204 is a separate assembly that is attached to body member 202 bywelding, as depicted at 206. An annular stabilizer sleeve 208 extendsgenerally around body member 202, and further extends to proximatecutting assembly 204 and forms the extended gage of the drill bit.Annular stabilizer sleeve 208 may be formed of steel or of another metalor metallic material, such as a tungsten carbide matrix, as known tothose skilled in the art. Annular stabilizer sleeve 208 includes aplurality of spiral stabilizer blades 34 extending to the gage dimensionof the drill bit, as established by the surfaces providing the maximumcutting diameter of the drill bit, as generally represented at 210. Astabilizer sleeve with straight, rather than spiral, blades may also beused. However, where the drilling operation will include geosteering,the spiral stabilizer blades are preferred as they will assureconsistent support of the drill bit relative to the low side of anon-vertical borehole, and thereby offer improved functionality for bothdrilling and resistivity measurements. Stabilizer sleeve 208 will alsotypically be coupled to body member 202 by welding; but in some casesmay also be attached by other conventional mechanisms such as beingattached by bolts or other insertable devices, as known in the art.

Body member 202 defines a central bore, indicated generally at 212,defined by a sidewall 214 and an inwardly projecting shoulder 216. Inthis example, a removable insert member 218 extends within central bore212, and includes a central web portion 228 extending between twosurfaces with external sealing assemblies, indicated generally at 220and 222. Sealing assemblies 220, 222 engage sidewall 214 and inwardlyprojecting shoulder 216, respectively, to define an annular cavity 226surrounding web portion 228 of insert member 218. Insert member 218 hasa central open passageway 224 that serves as the lower portion of a mudpassageway to nozzles proximate the bit face (not illustrated).

Annular cavity 226 provides a location for housing electronic circuitry,as indicated generally at 232. Drill bit 26 also includes an electrode234 which is positioned in an aperture 233 in a stabilizer blade 34 (andalso an aperture 235 in body member 202), such that the outer electrodesurface, at 236, extends proximate the outer contact surface of thestabilizer blade 34. In this example, electrode 234 is depicted as asingle button electrode, as opposed to a composite conductor electrodeas may be used for a focusing electrode, as generally depicted inFIG. 1. Electrode 234 will be insulated from stabilizer blade 34 andbody member 202 by an insulative insert or jacket 230.

Where generally continuous contact with the formation can be expected,then single button electrodes will be preferred, as they will providethe best measurements, particularly for formation imaging. Thecontinuous contact need not necessarily be by a single electrode, but insome configurations may be achieved by having at least one at a time ofa plurality of electrodes in contact with the formation. In suchconfigurations, one preferred implementation would include multipleelectrodes positioned radially around the drill bit, but at essentiallythe same longitudinal position on the drill bit. In situations whereeither the configuration of the drill bit or the formationcharacteristics are such that it may be expected to be difficult tomaintain formation contact with the electrodes, then focusing electrodeswill be better suited to drive current to, or receive the current from,the formation. Such focusing electrodes will be discussed in more detailin reference to FIG. 3. In many embodiments, electrode 234 will becoupled to electronic circuitry 232.

As noted in reference to FIG. 1, drill bit 26 also supports a toroidantenna 38. Toroid antenna 38 is supported by a protective mountingassembly indicated generally at 240, and is secured against a shoulder250 on the outer surface of body member 202. Protective mountingassembly includes a fiberglass insulative ring 242, a protective cover244, a stop ring 246, and a retention band 248. Fiberglass insulativering 242 extends generally around body member 202 to provide aninsulative support member for mounting the remainder of protectivemounting assembly 240. Stop ring 246 serves to help position toroidantenna 38 within the protective mounting assembly. The protective coverwill be formed of a non-conductive material, for example of fiberglassof a polyether ether ketone (PEEK), so as to not interfere with thefunction of toroid antenna 38. This protective mounting assembly isretained in position against stop shoulder 250 by retention band 248. Insome preferred implementations, retention band 248 will be a metal ormetallic member which is secured in place by heating the member toexpand it sufficiently to allow it to be placed in position adjacent theother described components of protective mounting assembly 240, andwhich will then shrink as it cools to form a secure and tight engagementwith body member 202 to secure the described components in the depictedpositions.

As shown in FIG. 2B, a passageway 252 extends through body member 202establishing a path for an electrical conductor 254, as well asappropriate insulative and sealing mechanisms, as depicted by sealingplug 256 having electrical conductors 254 extending therethrough (only aportion of an exemplary conductor is depicted, for clarity). Electricalconductors 254 will facilitate electrical communication betweenelectrical circuitry 236 and toroid antenna 38 through passageway 252.

Also shown in FIG. 2B is a connection block 260 that engages centralbore 212 in body member 202 above insert member 218. Connection block260 includes a generally centrally-located connection fixture 262,coupled to an annular support ring section 264 by a radially-extendingarm or web portion 266. Connection fixture 262 is thereby locatedproximate the center of the flow channel that connects with central openpassageway 224. Connection block 260 includes a passageway 272 whichextends between connection fixture 262, at a relatively uphole location,and a male connector portion 268, at a relatively downhole location.Male connector portion 268 has a sealing assembly thereon, andcooperatively engages a receiving bore 270 in body member 202, which iscoupled by a passageway 274 to annular cavity 226. A passage for one ormore electrical conductors is thereby formed extending from connectorportion 268 into cavity 226.

As noted previously, drill bit 26 couples to the lower end ofgeosteering assembly 22. Thus, geosteering assembly includes amechanism, such as a connection assembly 298 facilitating theestablishing of a mechanical and electrical connection with connectionfixture 262 in drill bit 26. Connection assembly 298 includes a mandrel280 that supports a block 282 proximate the lower end of mandrel 280.Block 282 may be retained in position by a suitable mechanism, depictedhere as one (of one or more) retention screws 284. Block 282 willinclude one or more web members to extend radially toward a centrallocation to support mechanical and electrical connectors. Thesemechanical and electrical connectors include an upwardly accessibleconnector, such as, for example a threaded coupling, indicated generallyat 286, that will cooperatively engage a lower connector 288 of aflexible electrical conductor assembly 290. Block 282 further includes adownwardly accessible connector, such as, again for example, a threadedcoupling 292 that will engage a threaded connection fitting 294 whichwill also threadably engage connection fixture 262. Thus, as a result ofthe electrical connection provided by connection sub 280, connectionfitting 294 and connection sub 261, electrical signals can be conductedbetween electrical conductor assembly 290 and electrical circuitry 236,and also to and/or from button electrode 234 and toroid antenna 38.

As will be known to those skilled in the art, a central passagewaythrough the drilling assembly, including the above-described components,will convey drilling mud to output nozzles proximate the face of thedrill bit, to lubricate and cool the drill bit, as well as to carrycuttings away from the bit face and uphole. Thus, as can be seen fromFIGS. 2A-B, the dimensions of the outer boundaries defining central openpassageway 224 change to compensate for the area occupied by, forexample, connection sub 280 and connection sub 261, so as to maintain agenerally uniform cross-sectional area to accommodate the fluid flow.Additionally, a tapered extension 296 can be found on the downstreamportion of connection fixture 262 to minimize any turbulence orcavitation that might otherwise be introduced into the mud flow.

The configuration of drill bit 26, and particularly of body member 202,present additional manufacturing considerations compared to moreconventional configurations. For example, in addition to theconsiderations present in the construction of any drill bit body member(or “bit upper section”) 202 requires at least one lateral aperture 235to accommodate some portion of electrode 234, or at least electricalconnections to electrode 234. Body member 202 may, of course, requiremultiple such lateral apertures where multiple electrodes or coils areplaced in blades 34 (as depicted at 36, in FIG. 1). Body member alsorequires a lateral aperture 252 formed to accommodate electricalconductors 254 (within an appropriate sealing plug 256) to communicatewith toroid antenna 38. Additional lateral apertures may be required toaccommodate insertable fasteners, such as either bolts or pins to securestabilizer sleeve 208 in position, if welding is not used to secure thesleeve to body member 202. Additionally, manufacturing of body member202 requires the forming of internal shoulder 216 in central bore 212.

As will be apparent to those skilled in the art, where PDC cuttingelements are used for the cutting assembly, the cutting assembly willtypically be formed of a matrix as described above. Additionally, itshould be recognized that stabilizer sleeve 208 may also be formedeither of a machined metal, or of a cast matrix. Where stabilizer sleeve208 is formed of steel or another metal, the electrode-receivingaperture(s) 233 in one or more blades 34 may be machined throughconventional techniques. Similarly, where stabilizer sleeve 208 is to becoupled to body member 202 by insertable fasteners (pins, bolts, etc.),the necessary apertures may again be formed through the conventionalmachining techniques.

However, where stabilizer sleeve 208 is formed of a cast matrix, thematrix material typically includes tungsten carbide (or another hardmaterial) that is infiltrated by an alloy, for example a copper-basedalloy. Components formed of such a matrix are not typically capable ofbeing machined in the same manner as metal components. As a result,where stabilizer sleeve 208 is to be formed of such a matrix material,it is preferred to cast the sleeve. This casting may be done throughtechniques generally known to those skilled in the art, in which a moldis constructed of an appropriate material (for example, sand, graphite,ceramic, etc.) for use in molding the component. The mold orsub-assembly, or the final part, may also be formed by any of severaltechniques, including building up through stereo lithography. As anotherexample, “sand printing” may be used, where a mold of the resultingshape is created, and then used as a positive form to create a reversemold formed of sand in an appropriate resin, that is ultimately hardenedin the desired conformity. That reverse mold may then be used to castthe matrix material. Another manufacturing alternative would be to castthe primary shape of stabilizer sleeve 208 in a conventional manner, butto then use non-traditional machining techniques to form the describedapertures, and potentially other conformities. An example of one suchnon-traditional machining technique is electrical discharge machining(“EDM”), where a series of electrical discharges between a toolelectrode and the matrix casting (acting as another electrode),typically in the environment of an intervening dielectric liquid (knownas “sinker” or “plunge” EDM), causes the incremental removal of materialbetween the electrodes.

Referring now to FIGS. 3A-B, those figures depict two alternativeconfigurations for focusing electrodes as may be used in drill bit 26,in place of the single element button electrode 234 depicted in FIG. 2A.FIG. 3A depicts a first focusing electrode configuration 300, having acentral electrode 302 surrounded by an insulator 304. Central electrode302 has a generally cylindrical shape (viewed from the exterior), andthus insulator 304 and subsequent structures in focusing electrode 304are concentric annular structures relative to central electrode 302.Extending around insulator 304 is a focusing electrode 306, againsurrounded by a second insulator 308. Similarly, the second focusingelectrode configuration 320 of FIG. 3B depicts a central electrode 322surrounded by a first insulator 324 which was then surrounded by afocusing electrode 326 and a second insulator 328. As known to thoseskilled in the art, a current applied to either of the focusingelectrodes 306, 326, and any current induced in the respectivesupporting structure (here, a portion of a stabilizing section blade 34)will serve to focus the measurement current applied to (or receivedfrom) the central electrode 302, 322 to extend more deeply into thesurrounding formation.

Referring now to FIG. 4, the figure depicts an upper portion 400 ofgeosteering assembly 22, as identified in FIG. 1, illustrated invertical section. As noted previously, in this example, toroid antenna20 is supported by the geosteering assembly. As an alternative, toroidantenna could be coupled to a separate sub interposed in drillingassembly 10, between the downhole motor and geosteering assembly 22.Because many different configurations of geosteering assemblies aresuitable for use with the described drilling assemblies, the specificconfiguration of the geosteering assembly will not be addressed here,other than to address one example of adaptations that may be made in ageosteering assembly to facilitate electric communication through theassembly. As an alternative to enabling those communications through useof a flexible conductor cable (44 in FIG. 1) as discussed herein, otherstructures may be utilized. As just one example, in systems configuredfor use with wired pipe, which will communicate signals throughconductors in the drillstring components, that same structure ofconductors might also be used to provide the desired electricalcommunication through the necessary components in the drill string,including the geosteering assembly. In this adaptation of thegeosteering assembly, structures are provided to engage an upperconnector 404 of flexible electrical conductor assembly 290 which willallow electrical communication from the top of the geosteering assembly(including toroid antenna 20) with toroid antenna 38 and electroniccircuitry 232 in drill bit 26, as previously described.

Upper portion 400 of geosteering assembly 22 includes a mandrel 402 thatcouples to an output shaft of a downhole motor for rotation with theshaft, and further includes a central assembly, indicated generally at406, that will rotate with mandrel 402. In the depicted system, mandrel402 and central assembly 406 cooperatively form an upper drive portionof the geosteering drive mechanism that will couple to drill bit 26.Mandrel 402 and central assembly 406 cooperatively define an annularrecess 408 that houses electronic circuitry 410 associated withoperation of the geosteering assembly. Central assembly 406 alsoincludes an upper connection block 412 with a connector 414 configuredto engage a complimentary connector from the downhole motor (notdepicted), and to facilitate electrical communication from the downholemotor (and above it) with the electronic circuitry 410 in geosteeringassembly 22, as well as the electronic circuitry 232 and toroid antenna38 of drill bit 26 (see FIG. 2A).

Connection block 412 includes an annular portion 416 that engages ashoulder 418 on an end portion 432 of central assembly 406, and thatsealingly engages the interior of mandrel 202. A web portion 420 extendsradially inwardly from annular portion 416 to support connector 414 in acentral portion of mandrel 402. Connector 414 is positioned bothradially and longitudinally to engage the complementary connector on themud motor (or any intervening components) when the tools are threadedtogether, so as to establish electric communication with the upholestructures. Connection block 412 also includes first and secondpassageways, 422 and 424, respectively. First passageway 422 extendsbetween connector 414 and a second connector 426 that is also preferablycentrally-oriented within mandrel 402. In some embodiments, connector414 and connector 426 will each be aligned along the longitudinal axisof mandrel 402 as that axis exists proximate the connectors. Secondconnector 426 is configured to mechanically and electrically engageupper connector 404 of flexible electrical conductor assembly 290. Aswill be appreciated by those skilled in the art, the exact structure ofupper connector 414 and its complimentary connector on the downholemotor (not depicted) and also of connector 426 and its engagingconnector 404 may be of any suitable configuration as known in the art,such as two assemblies which may be threadably joined, to establishmechanical engagement and to allow the engagement or other contact ofelectrical conductors to achieve the indicated electrical connections.

Passage 424 extends to electrical connector 420, which includes a firstconnector member 428 in connection block 412, and an engaging connectormember 430 supported by end portion 432 of central assembly 406. For theavoidance of doubt, certain passages as described and depicted herein(for example passage 424), consist of multiple bores, and are depictedto indicate the manner in which those bores might be actuallyconstructed. As will be apparent to those skilled in the art, once theidentified bores are formed, then undesired openings (for example, asindicated at 450) resulting from the machining operations will be closedby a plug or other appropriate structure, as is well known in the art.Engaging connector member 430 is in communication with a passage 434 incommunication with annular recess 408. Additionally, passage 434 extendsradially to the exterior of end portion 432 to an opening betweensealing assemblies which is aligned with a radial passage 436 extendingproximate the mounting location of toroidal antenna 20. Thus, passage424, electrical connector 426 and passage 436 facilitate such electricalcommunication as may be desired between toroidal antenna 20, electroniccircuitry 410, connector 414, and connector 426; and from connector 426,through electrical conductor assembly 290, to bit 26.

Toroidal antenna 20 may be coupled to the exterior of mandrel 402 by avariety of appropriate mechanisms. For example, toroidal antenna 20 andits associated components might be housed at least partially within arecess, or against a shoulder, formed in the exterior of mandrel 402, aswas depicted and described relative to the mounting of toroidal antenna38 on drill bit 26. As a depicted example of an alternativeconfiguration, toroidal antenna 20 is not housed within a recess, but isretained on the surface of mandrel 402 between two retention bands, at440 and 442. As discussed relative to bit 26, each of retention bands440, 442 may be a heat-expandable band that, when cooled, shrinks tosecurely engage the exterior of mandrel 402. As with toroidal antenna38, a non-conductive protective member 444, such as may be formed ofeither fiberglass or PEEK, will extend over antenna 20 and protect itfrom the exterior environment. Additional components (insulators,spacers, etc.), may be added as needed for a specific implementation. Tofacilitate the above-described electrical connection between toroidalantenna 20 and the other components, electrical conductors 446 will besealingly retained within passage 436 by extending through a sealingplug 448.

Referring now to FIG. 5, therein is depicted an alternativeconfiguration for a drill bit 500 suitable for use in a drillingassembly as described above in reference to FIG. 1. Drill bit 500 may beconstructed essentially identically to previously-described drill bit26, except that rather than a toroid antenna 38 extending in a planewhich is essentially perpendicular to the longitudinal axis through thedrill bit, drill bit 500 includes a conventional coil antenna 504 thatextends around the drill bit in a plane 506 that is angled relative tothe longitudinal axis 508 through drill bit 500. Coil antenna 504thereby provides a magnetic moment 510 that is not aligned with thelongitudinal axis 508 of drill bit 500. In some anticipatedconfigurations, coil antenna 504 may be disposed at an angle ofapproximately 45° relative to the axis through drill bit 500. However,many other alternative angles of the plane of the antenna may becontemplated, falling between perpendicular (as with toroid antenna 38)and greater than 0° (i.e., an angle of greater than 0 degrees, and nogreater than 90 degrees). In many configurations, an angle betweenapproximately 20 degrees and 70 degrees will be used. It is anticipatedthat the establishing of a magnetic moment at the drill bit which is notaligned with the axis of the drill bit should provide the capability toevaluate resistivity of the formations that the bit has not yetpenetrated, i.e. ahead of the drilling surface of the drill bit. Oneadditional relationship that should be noted is that because thedrilling assembly of FIG. 1 includes a drill bit that is deflected fromthe longitudinal axis of the drilling assembly (see axis 512 in FIG. 5;the deflection angle has been exaggerated for clarity of theillustration), even a coil antenna lying perpendicular to the axis ofthe drill bit (as with toroid antenna 38), with a magnetic momentaligned with the axis of the drill bit, will have a magnetic momentdisposed at an angle to the longitudinal axis of the drilling assembly.Where the coil antenna is disposed at an angle to the drill bit axis506, as depicted in FIG. 5, the antenna will also be located at an angleto the axis to the drilling assembly 512, though at a different angle.

In operation, when drilling assembly 10 is used in a drilling operation,the described components may be used to determine resistivity at thebit. In many operations this will include imaging of the formationssurrounding the well bore, and evaluating the presence of bed boundariesto be used in geosteering of the drilling operation. To obtain theneeded resistivity measurements, toroid antennas 20 and 38 will beselectively actuated to induce current in the drill string and in thedrill bit. That induced current, after passing through the formation,will be sensed through use the receivers directly on the drill bit,toroid antenna 38, or electrode(s) (as indicated at 36 in FIG. 1, or 234in FIG. 2). In most operations, particularly those including aconductive mud environment (typically, where water-based muds are used),the toroidal antennas will be alternately energized at frequenciesbetween 1 kHz and 20 kHz. While toroid antenna 20 will typically be usedprimarily, or exclusively, as a transmitter, toroid antenna 38 at thebit will, in some embodiments, be used as both a transmitter andreceiver. For example, Toroid antenna 20 will be used as a transmitter,and the resulting current will be received at Toroid antenna 38. Atother times, toroid antenna 38 will be used as a transmitter, and thecurrent induced in the formation will be received at the describedbutton electrode(s). That measurement may be used, for example, forimaging the formations at the bit. As will be apparent to those skilledin the art, although this discussion addresses one likely configuration,wherein toroid antennas 20 and 38 are each used as transmitters, and theelectrodes 36 are used as receivers, those functions conceptually couldalso be reversed between those components. Additionally, one or morebutton electrodes in the bit might also be used as a transmitter andthus appropriately energized; with the resulting current sensed by oneor more spaced electrodes on the drill bit. In that circumstance, thetransmitter electrode would preferably be longitudinally offset from thereceiving electrodes (such longitudinal offset between button electrodesis described in reference to FIG. 1). Such longitudinally offsetelectrodes may be used for measurements that are useful for depthcorrection. For example, separate images resulting from current receivedat two longitudinally offset electrodes may be compared to one another,and correlated in a desired manner to determine any needed depthcorrection. Additionally, button electrodes might each be replaced by arespective small coil antenna disposed in a stabilizing blade 34, as analternative structure for use in place of a button electrode. Thedetected currents will be measured and processed in a conventionalmanner to yield formation resistivity measurements, potentiallyincluding images of the formation, proximate the drill bit. In theexample of FIG. 5, when an angled coil antenna 504 is located on thedrill bit, it will typically be used only in a receiver mode, with thetoroid antenna 20 being used as the transmitter.

The advantages of this system include the ability to evaluate theformation right at the bit. While useful for many reasons, isparticularly desirable in geosteering applications where an appropriatecourse may be confirmed and/or corrected, as needed based on essentiallyreal-time information regarding the formations through which the bit isextending. In other applications, such as where relatively nonconductivedrilling muds are utilized, typically oil-based muds, it may bedesirable to operate the transmitters at much higher frequencies, in themegahertz range, to make the desired formation measurements.

Many modifications and variations may be made in the techniques andstructures described and illustrated herein without departing from thespirit and scope of the present invention. Accordingly, it should beclearly understood that the scope of the inventive subject matter. Isdefined only by the claims and their equivalents that are supported bythis specification.

We claim:
 1. A drill bit assembly, comprising: a body member, aplurality of cutting assemblies supported by the body member; anelectrode proximate an outer surface of the drill bit; and a releasableconnection assembly including at least one electrical conductor, theconnection assembly configured to engage an electrically conductingmember when the drill bit is coupled in a drilling assembly.
 2. Thedrill bit assembly of claim 1, further comprising an electrical antennacoupled to a portion of the body member.
 3. The drill bit assembly ofclaim 2, wherein the electrical antenna comprises a toroidal antenna. 4.The drill bit assembly of claim 2, wherein the electrical antennacomprises a coil antenna.
 5. The drill bit assembly of claim 4, whereinthe coil antenna extends in a plane at an angle of more than 0 degreesand less than 90 degrees relative to the longitudinal axis of the drillbit.
 6. The drill bit assembly of claim 1, further comprising anelectronics assembly retained within the drill bit, the electronicsassembly in electrical communication with an electrical conductor of thereleasable connection assembly.
 7. The drill bit assembly of claim 1,further comprising a removable insert defining the side wall of thefluid passageway through the drill bit, the removable insert furthercooperatively defining with the bit body a chamber within the drill bit,and wherein the electronics assembly is housed within the chamber.
 8. Adrilling assembly, comprising: a first tubular member supporting a firsttransmitter antenna; a steering assembly located beneath the firsttubular member; and an earth boring device comprising a drill bit, thedrill bit having a body member, a bottommost surface and a gage surface,and wherein the earth boring device comprises at least one of, a secondtransmitter/receiver antenna supported in spaced relation to the firsttransmitter antenna and within 36 inches of the bottommost surface ofthe drill bit, and at least one electrode generally at an outer surfaceof the drill bit.
 9. The drilling assembly of claim 8, wherein thesecond transmitter/receiver antenna is supported by the drill bit. 10.The drilling assembly of claim 8, wherein the earth boring devicefurther comprises at least one device selected from the group consistingessentially of a sleeve, a connection sub and a stabilizer, the devicecoupled to the drill bit.
 11. The drilling assembly of claim 10, whereinthe second transmitter/receiver is supported by the device.
 12. Thedrilling assembly of claim 8, wherein the first transmitter antenna is atoroid antenna.
 13. The drilling assembly of claim 8, wherein the secondtransmitter/receiver antenna is a toroid antenna.
 14. The drillingassembly of claim 13, wherein the second transmitter/receiver extendsaround a portion of the drill bit.
 15. The drilling assembly of claim 8,wherein the at least one electrode comprises a plurality of electrodes.16. The drilling assembly of claim 15, wherein electrodes of theplurality of electrodes are distributed radially around the drill bit,and are in at least two longitudinally spaced locations on the drillbit.
 17. The drilling assembly of claim 15, wherein the longitudinallyspaced electrodes are placed to be used for at least one measurementuseful for depth correction.
 18. A method of evaluating a formationduring a drilling operation, comprising the acts of: drilling theformation through use of a drill bit in a drilling assembly; inducing afirst current into a portion of the drilling assembly through use of afirst toroidal antenna at a first, relatively uphole location relativeto the drill bit; receiving and measuring a first current from theformation through a receiver on the side of the drill bit, the firstreceived current resulting from the first induced current; inducing asecond current into a portion of the drilling assembly through use of asecond toroidal antenna at a second location immediately proximate thedrill bit; receiving and measuring a second current from the formationthrough a receiver on the side of the drill bit, the second receivedcurrent resulting from the second induced current; determining at leastone resistivity parameter of the formation using the first and secondreceived current measurements.
 19. The method of claim 18, wherein thesecond antenna is supported by the drill bit.
 20. The method of claim19, further comprising the act of communicating signals from the firstantenna coil to the second antenna coil through a flexible electricalconductor extending through the drilling assembly and engaging aconnector within the drill bit.
 21. The method of claim 18, wherein thefirst and second received currents are both received at the sameelectrode.
 22. The method of claim 18, wherein the electrodes arecarried on at least one stabilizer blade of the drill bit.
 23. Themethod of claim 18, wherein the first antenna is a toroid antenna havinga magnetic moment aligned with the longitudinal axis of the drill bit.24. The method of claim 18, wherein the wherein the second antenna is acoil antenna having a magnetic moment extending at an angle relative tothe longitudinal axis of the drilling assembly, the angle being lessthan 90 degrees.
 25. The method of claim 24, wherein the drill bit isoriented at an angle to the longitudinal axis of the drilling assembly,and wherein the second antenna coil is further located at an anglerelative to the longitudinal axis of the drill bit, the angle also beingless than 90 degrees.
 26. A method of manufacturing a drill bit,comprising the acts of: forming an upper bit section, the upper bitsection comprising a central bore, and further comprising at least onegenerally radially extending passage, the passage extending from thecentral bore to the exterior of the upper bit section; securing acutting head to the upper bit section; and securing a stabilizer sleeveto the upper bit section, the stabilizer sleeve including an apertureextending from the exterior surface of at least one stabilizer blade tothe interior of the stabilizer sleeve, and wherein the aperture isaligned to be in communication with the radially extending passage ofthe upper bit section when the stabilizer is secured to the upper bitsection.
 27. The method of manufacturing a drill bit of claim 26,further comprising placing an insert within the central bore of theupper bit section, the insert having an annular wall defining a centralmud passage, and an annular chamber exterior to, and isolated from, thecentral mud passage.
 28. The method of manufacturing a drill bit ofclaim 27, further comprising securing an electrical antenna to theexterior of a portion of the upper bit section.
 29. A drilling assembly,comprising: a first tubular member supporting a first toroid antenna; asteering assembly located beneath the first tubular member; and an earthboring device including a drill bit having a body member, a bottommostsurface and a gage surface, the earth boring device comprising a secondantenna, the second antenna being a coil antenna extending around aportion of the earth boring device.
 30. The drilling assembly of claim29, wherein the second antenna extends in a plane disposed at an angleto the longitudinal axis on the drill bit, the angle being greater than0 degrees but no more than 90 degrees.
 31. The drilling assembly ofclaim 29, wherein the second antenna extends in a plane disposed at anangle to the longitudinal axis on the drill bit, the angle being between20 degrees and 70 degrees.
 32. The drilling assembly of claim 29,wherein the first toroid antenna is configured as a transmittingantenna, and wherein the second antenna is configured as a receivingantenna.
 33. The drilling assembly of claim 29, wherein the earth boringdevice further comprises electronic circuitry in electricalcommunication with the first toroid antenna.
 34. The drilling assemblyof claim 29, wherein the second antenna is supported on the drill bit.35. The drilling assembly of claim 29, wherein the earth boring devicefurther comprises at least one device selected from the group consistingessentially of a sleeve, a connection sub and a stabilizer, the devicecoupled to the drill bit.